The concept of depression of melting points by mixing two or more pure components is well known in the fields of alloying metals and organic chemistry. For example, when an organic material having a melting point of 200.degree. C. is mixed in a 50:50 ratio with another organic material having a melting point of 175.degree. C., the resulting mixture may very well have a melting point of 90.degree. C. This effect is often attributed to the interaction or mixing of the two components in the liquid state thus making it difficult for either of the constituents to assume their preferred crystalline or solid state. When considering organic compounds, the ones that tend to form very thermodynamically stable crystals experience the greatest depression in melting point especially when mixed with a compound having a different crystalline structure. For high molecular weight polymers, this depression is not very pronounced unless the polymer is crystalline. The family of polymers known as polyimides are well known to be amorphous and by mixing two different polyimides you would therefore not expect a significant lowering of either the softening or melting points.
One approach to preparing polyimides that does lend itself to the lowering of melting point concept is via the addition polymerization route. This process involves the preparation of an oligomeric imide having vinyl endcaps and generally represented by the formula: ##STR1## and where R is selected from aromatic amines such as ##STR2## where n=0, 1, 2, 3 . . .
When an oligomer of this type is heated it generally melts and polymerizes at the proper temperature through the vinyl linkage. ##STR3## The resultant polymeric material is highly crosslinked and is therefore thermoset. By polymerizing this type of material "in place" it is useful as an adhesive. This same processing method must be used for the preparation of fiber reinforced composites where the polymerization encapsulates the reinforcement and is set. Also this process is used whereby epoxies serve as adhesives and for composite matrix resins.
One particularly attractive addition-type polyimide that is commercially available is Ciba Geigy's P-13N which has the following structure: ##STR4## where n=1 or 2.
When this material is heated at 550.degree.-600.degree. F., the imide ring forms, liberating water, and the vinyl portion of the molecule undergoes polymerization to form a highly crosslinked, thermally stable polymer. This particular system has drawbacks in that the oligomer is a solid which makes for a very "boardy" prepreg when composites are to be fabricated since the resin system undergoes very little or no melt during the processing cycle. This lack of melt necessitates the use of extremely high pressure, for example more than 1000 psi, in order to fabricate consolidated composite parts. If the matrix resin were meltable at reasonable temperatures, the pressure could be considerably less and the process more readily useable.
There is thus a definite need in the art for a reduced temperature polymerization process for polyimide preparation.
Accordingly, an object of the present invention is provide an improved process for preparing addition type polyimides that may be employed in a "hot melt" prepregging without the use of additional solvents.
Another object of the present invention is a process for reducing the polymerization temperature of addition type polyimides.
Another object of the present invention is a novel polyimide having a reduced melting temperature.
Another object of the present invention is a novel process of preparing an improved prepreg for use in fabricating composite structures.